The present invention relates to a deflection yoke provided on a CRT (Cathode Ray Tube) such as a television set or computer display, and particularly to a deflection yoke including a bobbin having a shape suitable for forming a deflection coil into a saddle shape.
With the trend toward higher image qualities of television sets and the like, deflection yokes have been improved to perform double-speed deflection for realizing the higher image qualities.
In this case, as a result of doubling the deflection frequency, the inductance of a deflection coil is required to be reduced to a quarter and an applied current is required to be increased. This causes problems associated with power consumption of a TV circuit and heat generation of the deflection yoke.
To meet with the requirement to double the deflection frequency, there has been proposed a method of doubling a voltage of a horizontal deflection pulse.
In a usual TV set, the voltage of a horizontal deflection pulse is generally specified at 1.2 kV from the viewpoint of the withstand voltage of a transistor, and accordingly, the doubled voltage of a horizontal deflection pulse becomes 2.4 kV.
When the deflection frequency is doubled and the voltage of a horizontal deflection pulse is doubled, the inductance of the deflection coil may be equal to that specified in accordance with the NTSC form, and the current amount may be reduced to a half that required for the related art double-speed deflection, that is, may be equal to that specified in accordance with the NTSC form.
The increased voltage of a horizontal deflection pulse is effective to reduce the power consumption upon high speed scanning and to suppress an adverse effect of the winding accuracy exerted on image quality due to reduction in inductance.
When the doubled voltage is applied to the deflection coil as described above, however, there arises a problem that corona occurs among respective wires of the coil. Corona means firing discharge occurring before spark discharge, which results from partial dielectric breakdown caused at a portion where a surface electric field is locally large when electric fields of conductors are not equal to each other.
The occurrence of corona may cause layer short-circuit with elapsed time, which leads to a serious problem associated with fuming and firing.
A deflection yoke employing a saddle-shaped deflection coil is classified into a type in which the deflection coil is wound around a die (hereinafter, referred to as "a die-winding type"), and a type in which the deflection coil is wound around a bobbin (hereinafter, referred to as "a bobbin-winding type").
In the die-winding type, wire-winding is performed around the outer periphery of a die, followed by current-carrying, molding and cooling to form the coil shape. In this case, to mold the coil, the wires are bound by filling gaps among the wires with an insulating fusion agent (for example, Japanese Patent Laid-open No. Sho 59-119640 discloses a method of manufacturing a saddle-shaped deflection coil of a deflection yoke of a die-winding type).
In the bobbin-winding type, a saddle-shaped deflection coil is formed without molding the coil by winding wires on a bobbin along grooves or ribs previously provided in or on the bobbin.
The bobbin used for the bobbin-winding type is an integrally molded product from a heat-resisting synthetic region (i.e. an insulating) or the like. The bobbin has a main body portion, formed into an approximately horn-like shape, for covering the outer periphery of a cathode ray tube in a range from a funnel portion to a neck portion, and groove portions integrally formed at both end portions of the main body portion in the direction of the tube axis or Z-axis.
A main coil portion of the saddle-shaped deflection coil is wound on the main body portion of the bobbin in the direction of the tube axis of the cathode ray tube. To guide the winding of respective wires of the main coil portion, a plurality of ribs are formed on the inner surface of the main body portion in the direction of the tube axis.
The groove portions are formed at both the end portions of the main body portion in the direction of the tube axis into annular shapes centered on the tube axis. A bent portion of the saddle-shaped deflection coil is wound on each groove portion in the direction around the tube axis.
In such a bobbin-winding type, the coil can be wound around the bobbin, and therefore, it is not subjected to current-carrying and molding after winding.
In the above deflection yoke of the bobbin-winding type, occurrence of corona in the horizontal deflection coil has been prevented as follows. With respect to the main coil portion of the horizontal deflection coil, there has been adopted a method of restricting the positions of respective wires of the coil by, for example, forming slits or the like in the bobbin.
With respect to the bent portions of the horizontal deflection coil, there has been adopted a method of changing the thickness of each bent portion or tapering the thickness of the bent portion disposed on the front surface side of the cathode ray tube.
In addition, an example in which the bent portion is displaced in the direction of the tube axis has been disclosed, for example, in Japanese Utility Model Publication No. Sho 57-34673.
In the above method, however, there arises a problem that the overall length of the bobbin becomes longer. Also, when the thickness of the bent portion is thinned, there can be obtained an effect of preventing occurrence of corona; however, there arises a problem that the outer shape of the bent portion becomes larger. In addition, a method in which projections are provided in part of the groove portion has been disclosed; however, such a method is undesirable from the viewpoint of practical use.
More over in the bobbin-winding type, since the positions of the wires are not particularly restricted in the groove portion of the bobbin, there occurs disorder of the wires in the groove portion, with a result that gaps are formed between the wires.
By the formation of fine gaps between the wires in the groove portion of the bobbin, corona is liable to occur thereat as will be apparent on the basis of the principle to be described later, thereby making it impossible to increase a corona inception voltage.